Time-multiplexed data channels are currently enjoying a wide variety of applications. Typically such time-multiplexed data signals serially transmit a number of channels with each channel having a dedicated time slot within a frame. Further, the time slot within a frame of each channel is identified with reference to a marker known as a framing bit having a dedicated bit location within the frame. For instance, a Bell System standard format known as DS-1 is used to transmit 24 separate channels of pulse code-modulated voice signals or digital data at 1.544 MHz. Each channel is allocated a time slot for 8 bits during each frame. All 24 channels are grouped together to form a group of 192 bits and each group of 192 bits is preceded by a framing bit. Together, the 193 bits make up a frame and 12 frames are grouped together to form a superframe. In addition to the framing bit, signaling bits are included within the framing structure. However, the signaling bits vary from one specification to another.
The 12 framing bits within a superframe define a framing pattern. The Bell System standard known as D2/D4 framing uses a standard framing pattern as set out below in Table 1, where a terminal framing bit F.sub.T alternates with a signaling framing bit F.sub.S.
TABLE 1 ______________________________________ S-BIT FRAME TERMINAL SIGNALING NUMBER FRAMING F.sub.T FRAMING F.sub.S ______________________________________ 1 1 -- 2 -- 0 3 0 -- 4 -- 0 5 1 -- 6 -- 1 7 0 -- 8 -- 1 9 1 -- 10 -- 1 11 0 -- 12 -- 0 ______________________________________
In order to decode information in the serial channels, the framing bits must be isolated. This problem of isolating frame codes requires complicated circuitry that must be adapted to a particular frame code to be effective. Because the Bell System standard D2/D4 framing code has gained widespread acceptance, Rockwell International has produced a single integrated circuit known as the R8060 T-1 receiver for detecting the D2/D4 framing code. (See T-1 Primer, Document No. 219300N10, Rockwell International, Feb. 1984).
However, other framing patterns are in widespread use. For instance, a standard known as DMA-1 is implemented by placing alternatively one and zero in consecutive framing bit locations.
Because of the availability of the Rockwell R8060 T-1 receiver and related circuitry, it is desirable to modify framing patterns such as the DMA-1 so that they can be received using the Rockwell standard chip. Prior methods for detecting the non-standard framing patterns require large numbers of integrated circuits and are relatively time-consuming. For instance, in one method, circuitry starts at an arbitrary bit location within a data stream and searches from this arbitrary location forward, checking each consecutive bit location to detect the frame code pattern. This forward checking method requires approximately 40 integrated circuits to accomplish and can take a large amount of time to locate the framing bit. Further, the time it takes to locate a framing bit is extremely dependent on the type of data in the non-frame bit locations.
A second method of frame code detection that could be adapted to detect frame codes works by taking 8 frames of data and storing it in a 193 bit by 8 matrix. Each 8-bit row in the matrix is then searched for the framing pattern. This second method still suffers from the fact that a large number of integrated circuits are required. However, it greatly reduces the time required for determining frame bit locations.
The expense of creating and manufacturing a circuit for detecting non-standard framing codes is amplified by the need to be compatible with more than one type of framing code. Accordingly, it is desirable to have a system which can economically detect more than one format of framing codes.